Alcohol and drug abusing and dependent delinquents represent a greatly under served population that is at high risk of presenting significant deleterious long-term outcomes and costs (emotional, physical, financial) for themselves, their families, communities, and society. In designing treatment services for this population, it is critical to address the empirically determined contributors to alcohol and drug abuse in adolescents. Specifically, extensive and sophisticated research findings support the view that adolescent alcohol and drug use and abuse are multi-determined, linked directly or indirectly with cognitive/emotional characteristics of youths and key aspects of the family, peer, school, and neighborhood systems in which youths are embedded. Short-term and long-term outcomes from eight randomized trials of multisystemic therapy demonstrate that MST holds considerable promise in the treatment of serious clinical problems in adolescents and their families. Recognition of such promise is becoming widespread among leading academics and policy makers in the fields of substance abuse,juvenile justice, and mental health. Such promise is due largely to the capacity of MST to provide individualized and comprehensive services that overcome barriers to service delivery and directly address the multiple factors linked with antisocial behavior in the natural environments of youths and families. With regard to the treatment of alcohol abuse and dependence, four studies of MST have demonstrated at least short-term reductions in alcohol and drug use with adolescents presenting serious clinical problems. In light of these encouraging but not definitive results, this application proposes a rigorous study of the effectiveness of MST (plus an enhancement that integrates key components of the Community Reinforcement Approach under conditions of high treatment fidelity. Specifically, in a 2 (treatment condition: MST vs. current community services [CS]) x 4 (time: pretreatment, posttreatment, 6-month follow-up, 12-month follow-up) design, 144 juvenile offenders who meet DSM-IV criteria for alcohol abuse or dependence will be randomly assigned to MST versus CS treatment conditions. A comprehensive multimethod, multisource evaluation will address the following aims: Aim 1. To evaluate MST treatment effects on adolescent alcohol and drug use, criminal activity, and mental health functioning; as well as on family relations, peer relations, and school attendance. Aim 2. To track substance abuse and mental health service utilization and their associated costs across the treatment conditions to the 12-month follow-up. Aim 3. To test hypotheses about the mediators and moderators of MST treatment effects.